In-wheel motor drive units are advantageous due to their small environmental loads as they are driven by electricity, and are also advantageous as they are mounted in wheels of automobiles to drive the wheels and thus a larger interior space can secured as compared to engine automobiles. For example, a technique of suspending an in-wheel motor drive unit from a high-mount double wishbone suspension device is proposed as a structure for attaching an in-wheel motor drive unit to a suspension device on a vehicle body (see Non-Patent Literature 1).
Non-Patent Literature 1 specifically discloses a coupling structure between an in-wheel motor drive unit and a high-mount double wishbone suspension device. In a typical high-mount double wishbone suspension device for use in engine vehicles, a steered wheel is attached to a knuckle and the knuckle is coupled to a lower arm and an upper arm, as described in, e.g., Japanese Unexamined Patent Application Publication No. 2005-178410 (Patent Literature 1). In view of Patent Literature 1, the in-wheel motor drive unit is attached and fixed to a knuckle in Non-Patent Literature 1.
Moreover, the applicant has already proposed a technique described in Japanese Unexamined Patent Application Publication No. 2010-116017 (Patent Literature 2). In the technique of Patent Literature 2, a casing of an in-wheel motor drive unit is provided with seats that are coupled to a trailing arm of a suspension device. Each seat has a flat surface facing downward and a threaded hole formed in a direction perpendicular to the flat surface. Bolts inserted through the trailing arm are screwed into the threaded holes, so that the trailing arm closely contacts the flat surfaces of the seats and the suspension device is coupled and fixed to the trailing arm.